Fluidic oscillators are well known and document WO 93/22627 gives an example which is shown in plan view in FIG. 1.
That oscillator 1 which is symmetrical about a longitudinal plane of symmetry P comprises an oscillation chamber 3 and an obstacle 5 housed inside the chamber.
The obstacle 5 has a front wall 7 in which a "front" cavity 9 is formed facing an opening 11.
The opening 11 defines a fluid inlet into the oscillation chamber 3 and it is suitable for forming a two-dimensional fluid jet that oscillates transversely about the longitudinal plane of symmetry P of the oscillator.
During operation of the fluidic oscillator, when the fluid jet encounters the front cavity 9 and sweeps over it during oscillation, main vortexes T1, T2 form on either side of the jet (FIG. 1) and alternate between being strong and weak, in phase opposition, and in relationship with the oscillation of the jet.
In FIG. 1, the vortex T1 occupies space that is much greater than the space occupied by the front cavity of the obstacle, and the pressure of this vortex is such that the jet is deflected into an extreme position in spite of the presence of the other vortex T2 located between the front wall 7 of the obstacle 5 adjacent to the cavity and the wall 13 facing the oscillation chamber and connected to the opening 11.
When the fluid jet is in this position, a portion of the flow from the jet is directed downstream from the obstacle, and another portion feeds the vortex T2 which grows larger and larger and whose pressure increases up to the moment when the pressure is sufficient to cause the jet to change over to the other side, into the opposite extreme position.
The jet thus oscillates from one extreme position to the other, and by detecting the frequency of oscillation of the jet, it is possible to determine the flow rate of the fluid, with the frequency being considered as being proportional to the flow rate.
To reduce errors in determining the fluid flow rate, the ratio of frequency of oscillation to flow rate must not vary too much as a function of flow conditions.
Unfortunately, under so-called "transition" conditions, i.e. when the Reynolds' numbers calculated for the flow at the opening 11 are situated at around 300, the Applicant has observed that a high pressure zone (vortex T3) appears in the vicinity of the base of the fluid jet on the side where the vortex T1 is to be found, together with other localized vortexes facing the front wall beneath the vortexes T1 and T3 in FIG. 1.
These vortexes reinforce the action of the vortex T1, and as a result, more time is required by the vortex T2 to acquire enough force to counterbalance the pressures exerted by T1 and T3, thereby reducing the frequency of oscillation and thus giving rise to errors in fluid flow rate determination.
Also, document U.S. Pat. No. 4,244,230 discloses a fluidic oscillator having a nozzle which extends towards a U-shaped obstacle defining an oscillation chamber. The longitudinal size of the side walls of the nozzle is equal to or greater than the distance between the ends of the walls of the obstacle and the apex of the downstream surfaces of two elements of semi-oval section disposed perpendicularly relative to the duct and whose main axes are parallel to the flow direction of the fluid. During operation of the fluidic oscillator, that type of nozzle affects the oscillation of the jet by considerably impeding the development of the vortex T1.